In a saucepan, heat the milk over medium heat.
In a separate bowl, whisk together the eggs, sugar, butter, maizena, flour, salt, and vanilla extract.
Gradually add the mixture to the warm milk, whisking continuously until it thickens. Be careful not to burn the mixture. Once thickened, set aside to cool completely.
Heat a cast iron jaffle iron on a gas burner and spray both sides with non-stick spray.
Arrange half of the buttered bread slices on a flat surface, butter side facing out.
Spoon an even amount of cooled milk tart filling onto each slice. Cover with the remaining bread slices, butter side out.
Place inside the jaffle iron, close, and trim off excess crusts.
Toast on each side until golden brown with even grid marks.
Roll the hot jaffles in the cinnamon sugar mixture for a delicious final touch.